1. Field of the Invention
The present invention relates to a sintered mullite-based ceramic body and a process for manufacturing same. The mullite-based ceramic body is particularly suitable for ceramics used in the electronics field.
2. Description of the Related Art
Due to recent trends toward an increase of the packaging density and a reduction of the production costs of electronics parts, materials of packages and substrates are required to have a lower dielectric constant, a higher thermal conductivity, a higher mechanical strength, and a thermal expansion coefficient close to that of a semiconductor element, and to be able to be manufactured at a lower cost. Further, a highly reliable gas sealing property (hermeticity) is required. In attempts to satisfy these requirements, aluminum nitride ceramics, silicon carbide ceramics, mullite ceramics, and low-temperature-fired ceramics, etc., have been investigated and developed.
Among the above ceramics, aluminum nitride ceramics and silicon carbide ceramics have thermal conductivities, thermal expansion coefficients and mechanical strengths superior to those of the alumina ceramics now widely used in the electronics field, but have higher dielectric constants, and thus disadvantageously lower the operation speed of high speed integrated circuits.
Low-temperature-fired ceramics comprising ceramics and glass, i.e., glass ceramics, have a thermal expansion coefficient matching that of thus, so that heat stress between a semiconductor element and a substrate is reduced, and they can be designed to have a dielectric constant as low as 3 to 7. However, the low-temperature-fired ceramics disadvantageously have a low thermal conductivity remarkably lower than that of alumina ceramics, and thus have a low cooling effect.
These ceramics can be used in combination with metals having a low electrical resistance, such as silver and gold. HOwever, these metals cause problems in that, for example, silver brings on the problem of electromigration, restricting the applications thereof, and gold is very expensive. Further, materials that can be soldered to these metals at a temperature lower than that used for firing these ceramics are restricted.
On the other hand, mullite ceramics have a low dielectric constant and a low thermal expansion coefficient on a par with those of low-temperature-fired ceramics, and have a high thermal conductivity superior to that of low-temperature-fired ceramics. Also, mullite ceramics have an advantage in that metallization, the soldering of outer leads, and the formation of electrical resistance elements by post firing can be reliably conducted by conventional processes used for alumina ceramics.
A sintered mullite body is conventionally produced by firing, at about 1600.degree. C., a mixed powder of alumina (Al.sub.2 O.sub.3) and silica (SiO.sub.2), which are the constituents components of mullite. However, the resulting sintered body has a low mullite yield, and thus, the mechanical strength and humidity resistance of the body are low. To increase the mullite yield, kaolin or Mokusetsu clay (which is similar to Ball clay) are incorporated into the starting material of a mixture of alumina and silica. But, in the cases, these clay minerals emit a high amount of .alpha.-rays, which inhibits the use of the sintered body as a semiconductor device packaging material. These old-type mullites have been used as a raw material for refractories, and in this case, a certain amount of sodium oxides in the body is required to obtain a high creep resistance of the refractories. Nevertheless, these alkaline earth impurities, such as sodium oxides, must be eliminated to satisfy the above electric requirements.
On the other hand, mullite powders produced by an electrofusion method, etc., produce a high mullite yield in a sintered body, which is therefore useful as packaging material for electronic parts or substrates of semiconductor elements. The green body consisting of these mullite powders generally cannot be sufficiently densified by firing at a temperature lower than 1600.degree. C. A sintered body manufactured from a mixture of such electrofused mullite powders and an alkali earth metal oxide as a sintering agent is known. However, the obtained sintered body does not have sufficient mechanical strength, and other problems often arise if an electrical resistance element is formed on the body. In the latter case, these problems include a deterioration of the resistance characteristics and an adhesion to the body, of the electrical resistance element. A sintered body manufactured from a mixture of the above mullite powders and an alkali earth metal and zirconia is also known. This sintered body has a high mechanical strength, but emits .alpha.-rays due to the zirconia content, which may cause errors during the operation of semiconductor elements.
As another body having a high mechanical strength, a pure mullite ceramic is already known as a structural ceramic. This sintered body is obtained only by firing at a temperature higher than 1650.degree. C. Additionally, this sintered body does not include a glassy phase, and thus does not permit an adequate adhesion strength of co-fired conductors when formed on the surface thereof.
Sometimes the packaging material for semiconductor devices is required to have a light-shielding property, to prevent operation errors of a semiconductor element due to entrance of light in the low wavelength range, including ultraviolet rays. Note, all of the above mullite ceramics are white and have a poor light-shielding property.